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Let's say, whether for magical or scientific reasons, Earth and the moon are separated from their star, the Sun, and are sent hurtling through space.

Ignoring the reasons for such an event, how could humans survive for at least 1,000 years, if they could survive at all?

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    $\begingroup$ They don't. End of story. $\endgroup$ – jamesqf Jul 6 at 4:47
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    $\begingroup$ I hope your space habitat technology is well developed and you've found a way to fuel fusion power from non-exotic elements, because you're basically going to be living on an giant, frozen asteroid for the next thousand years. Then in a thousand years you die because a thousand years isn't long enough to get anywhere useful. Unless you have interstellar spaceships. Then you just leave Earth. $\endgroup$ – DKNguyen Jul 6 at 5:01
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    $\begingroup$ Reminds me a bit of "A Deepness in the Sky" $\endgroup$ – Dragongeek Jul 6 at 7:49
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    $\begingroup$ @Dragongeek, ...or "Space 1999" only with frozen earth hauled along behind the sparsely inhabited moon. This actually would work better than the original series because they could scavenge atmosphere, supplies and geothermal energy from the lifeless planet while continuing to live in relative safety within the moonbase. $\endgroup$ – Henry Taylor Jul 6 at 8:01
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    $\begingroup$ @Dragongeek: See also Fritz Lieber's short story A Pail of Air (1951). $\endgroup$ – Michael Seifert Jul 6 at 14:05
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If there is little or no warning, there is no chance of survival. However, with some preparation, it is feasible to survive deep underground - a few km would do.

The interior of the Earth is quite warm, to the extent that for every km you go underground, the temperature rises 25–30 °C (the geothermal gradient). Earth's internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, and latent heat from core crystallization. The sun only heats the top few metres of the Earth's surface. Hence, keeping warm is not a problem. There is also plenty of water underground in aquifers, which can go as deep as 9 km or more. Air can be recycled, but it will still be present on the surface as frozen air, which can be mined.

For energy, geothermal energy is an obvious solution. Nuclear power could also serve, though storing the nuclear waste might be a problem. Ejecting it to the surface is no good if you want to live there after only 1,000 years.

With heat and energy, you can grow crops under artificial lighting and even keep a few animals like dairy cattle, chickens, and perhaps sheep for wool, milk, and meat.

Some places may be more suitable than others, like Iceland, which has many hot springs that provide ready heat and energy near the surface. You would only need to dig down far enough to have protection from meteorites once Earth's atmosphere freezes.

The greatest challenges for survival for a thousand years will be to maintan the knowledge to maintain the machinery that keeps underground cities running and to preserve biodiversity and genetic diversity. The latter could feasibly be managed by storing seeds and fertilized eggs in permafrost near the surface.

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    $\begingroup$ Dumping nuclear waste to the surface is no problem at all. The surface is going to be a frozen desolate wasteland that is useless to us. We're not going to be living on it or even journeying to it outside of what will basically be a space suit. $\endgroup$ – StephenG Jul 6 at 11:14
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    $\begingroup$ Having a surface at air-freezing temperatures would be the dream of any power plant engineer: You can get rid of entropy without sacrificing much eneregy, allowing you to build 90% efficient power plants. Even geothermal energy with a source of, say, 200°C suddenly starts looking efficient when your heat sink is at -200°C. $\endgroup$ – cmaster - reinstate monica Jul 6 at 13:58
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    $\begingroup$ Nuclear waste, even for 1000 years will be a really small amount. We're talking about 1000m3 of unprocessed waste per power plant, assuming technology does not improve (and remember you can still process it). And you don't even have to maintain fuel pool. $\endgroup$ – spectras Jul 6 at 14:07
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    $\begingroup$ @StephenG To visit the surface would probably even require something more complex than a standard spacesuit--you'd be navigating large drifts of "snow" which means the suits would require heaters far more powerful than what Apollo or current EVA suits have. $\endgroup$ – Dragongeek Jul 6 at 14:12
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    $\begingroup$ The concern isn't too much nuclear waste, it's too little nuclear fuel. A typical 1000 MW reactor needs 25 tonnes of enriched fuel per year or 25,000 tonnes for 1000 years. Fuel which needs to be stored and will degrade. The reactors can be smaller, more efficient, and make use of breeder reactors to stretch their fuel, but once the fuel is gone, it's gone. Ideally nuclear power would be used initially while they develop a renewable source. $\endgroup$ – Schwern Jul 6 at 17:13
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With the Sun gone, so is the motor of the food chain, if we exclude the life forms thriving around the volcanic chimneys deeps in the ocean. But tubular worms are yet to become a staple of our breakfast.

Moreover with the Sun gone our atmosphere will turn into a layer of iced gases, laying on top of our frozen oceans.

The only way to survive would therefore be to have an abundant and cheap energy source to allow humans growing plants, keep their shelters warm and supply their industry which has to produce literally everything that is needed, as nothing will be spontaneously available in nature.

I think this calls for nuclear fusion, integrated here and there by geothermal power.

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    $\begingroup$ The abundant and cheap energy source would also have to be available immediately, depending on the warning received about the event, because there'd be no time to build generators or shelters. $\endgroup$ – jdunlop Jul 6 at 7:15
  • $\begingroup$ @jdunlop As would the facilities to use that energy to grow food and recycle resources en masse. $\endgroup$ – DKNguyen Jul 6 at 22:01
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Given a few years of warning, humans could survive in bunkers deep beneath the ground, relying on nuclear power to heat their habitats and run the grow lights in their farms. Not everyone could be saved, in fact the vast majority would die. But a few hundred thousand, maybe 1 million, could take refuge in underground settlements all around the world.

Nuclear fusion would be nice, but isn't actually necessary. Given such drastically reduced energy demands, there is more than enough fissile material to power a society like this for millennia, especially if supplemented with geothermal power.

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I think the question is interesting, but the phrase "...Earth and the moon are separated from their star, the Sun, and are sent hurtling through space." is rather vague.

My response assumes "hurtling" means "at much less than the speed of light" and that the laws of physics still apply. The key is escape velocity. It could be weeks before the Earth exits the Godilocks Zone, and more weeks before significant freezing takes place. There is time, therefore to begin crash programs to dig. As the Earth freezes from the outside in, digging has to keep head of freezing.

I'm going to ignore the moon for the purposes of this response, although the existence of a permanent base on the moon would definitely be a good plot line.

Given my assumptions and the current level of technology (The Boring Company), I do think that it's quite feasible to save a significant portion of the Earth's current population.

You have an interesting idea here. I'm looking forward to your first novel.

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    $\begingroup$ Whether you could dig a big enough hole is rather pointless, our current food production isn't at all set up for such massive hydroponics. Our experiments with creating a closed bio-cycle have not fared well. $\endgroup$ – SoronelHaetir Jul 6 at 15:39
  • $\begingroup$ I doubt you could even accomplish this with a decade of preparation with current tech. $\endgroup$ – eps Jul 6 at 18:19
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Jupiter still exists?

If so, lets play an Arthur C. Clark and turn it in a new sun, just like in 2001.

In his story they use monolith magic, i say let's go with the simple route and build nukes.

Big ones.

enter image description here

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    $\begingroup$ The Earth is hurtling away from and out of the solar system, not the sun has gone out. $\endgroup$ – DKNguyen Jul 6 at 22:02

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